The present invention relates in general to a control device for a variable displacement pump and in particular to a variable displacement pump, the adjusting element of which for controlling the volume of flow is controlled by a pressure control valve which applies and/or releases pressure medium to or from the adjusting element in response both to the pressure existing in the pressure line of the pump and an adjustable control pressure as a reference input value.
In machines having hydraulic servos, particularly in the field of plastic injection molding machines, there is a necessity to produce a large flow and high pressure from a relatively low level pressure and reduced flow across the load within the shortest possible period of time; such high pressure is subsequently to be maintained with a minimum amount of flow. Such conditions, for example, may occur in blow forming machines where the two mating parts of the blow mold are first to be closed whereupon such closure is to be maintained under high pressure.
It has been suggested to arrange a remote control pressure relief valve in the pressure line of the pump. This arrangement, however, exhibits the drawback that the power consumption of the pump will be exceedingly large when the pressure line is highly pressurized because the entire flow of the pump will bypass into the tank via the pressure relief valve across which the system pressure is relieved.
It is, therefore, advantageous to select a variable displacement pump of the type where the flow may be adjusted by a remote control pressure valve. The remote control pressure valve reduces the flow of the variable displacement pump to the level necessary for maintaining the pressure. This arrangement is well adapted to the control of high pressures; it makes it, however, very difficult to achieve zero or idle stroke pressures within a range of 10 bars or less.
Moreover, it is impossible to adjust the variable displacement pump from a low level pressure position into the maximum flow position in a very short period of time. The cause may be seen in the fact that with the very low level pressure in the pressure line, no hydraulic force will be available for displacing the pump. A further drawback is that neither the cooling nor filtering systems will effectively operate in the idle stroke position of the pump.